The invention relates to process control and, more particularly, to improved deadtime process control apparatus and methods.
"Process control" refers to the control of the operational parameters of a process by monitoring one or more of its characteristics over time. It is used to ensure that the quality and efficiency of a process do not vary substantially during a single run or over the course of several runs. Process control has application in both the manufacturing and service sectors.
A process control unit, or "controller," typically operates by monitoring and comparing a process characteristic, the controlled variable, with a desired "setpoint" level to determine whether the process is operating within acceptable bounds. As the controlled variable begins to deviate, the controller manipulates one of the process inputs, the manipulated variable, to bring the process back to the desired level of activity.
For example, as shown in FIG. 1, a process controller can oversee a process in which fluid flows at a constant rate from a continuously refilled tank. The controller monitors the liquid level in the tank and, when necessary to prevent the tank from running dry or overflowing, adjusts an inlet valve to increase or restrict the inflow.
Among the controllers developed by the art are those in which the manipulated variable signal is generated as a predetermined mathematical function of the controlled variable. One such controller, illustrated in FIG. 2a, is the proportional-integral-derivative (PID) controller. There, the manipulated variable is generated as a function of the "error," that is, the difference between the controlled variable and the setpoint: ##EQU1## where, m is the manipulated variable;
e is the error; PA1 P, I and D are constants PA1 .tau..sub.f is the filter time constant; PA1 .tau..sub.d is the process deadtime; PA1 .sigma..sub.m is the noise level in the controlled variable signal; and PA1 .sigma..sub.set is predetermined noise level.
In addition, the art has developed model-based controllers which model, mathematically, the operation of the process. One such controller is the deadtime controller, which relies on a model of the process deadtime and lag to determine values for the manipulated variable.
Deadtime is the time it takes a change in the manipulated variable to be reflected by a change in the controlled variable. For example, in a paper making process, deadtime is the time it takes for a change in the bleaching agent added to the initial slurry to be detected by a photo sensor that measures the whiteness of the final product web.
Lag is the time, after the deadtime period, that it takes the controlled variable to move approximately 63% of its final value, following a step change in the manipulated variable.
A deadtime controller of the type referred to above is shown in FIG. 2b. It is constructed by adding a "deadtime element," i.e., a time delay, into the integral feedback loop of a PID controller. This controller is referred to by the mnemonic "PID.tau..sub.d."
Prior art PID.tau..sub.d controllers have not proven entirely satisfactory when utilized to control deadtime-dominant processes, that is, processes whose output characteristics are primarily determined by deadtime (as opposed to lag).
It is, accordingly, an object of this invention to provide improved methods and apparatus for process control. More particularly, an object is to provide improved PID.tau..sub.d deadtime controllers.
Still another object is to provide a deadtime controller capable of accurately and efficiently controlling deadtime dominant processes.
These and other objects are evident in the description which follows.